This invention relates to a furnace for solid fuels, in which the fuels are charged into a combustion chamber by at least one conveyor screw. The combustion chamber is charged simultaneously with combustion air by at least one blower.
In furnaces of so-called underfeed construction, solid fuels, particularly combustible wastes such as, for example, garbage, wood shavings, bark shavings and similar shavings are pressed by means of a conveyor screw or a charging piston into an elongated grate trough. Because of a continuous replenishing charge of fuel, the fuel rises through a fire grate arranged above the trough, and is subjected to combustion with an additional supply of atmospheric oxygen. In this case, the combustion air is blown laterally into the furnace through a lower blast or wind box to provide a lower air supply or below-grate blast, and through injection nozzles arranged above the fire grate or the glowing fire to provide an upper air supply.
Furnaces of the above type, usually intended for small capacities, have various deficiencies. For example, the conveyor screw cannot press the fuel material from the trough, through the fire grate and toward the top of the furnace with uniform speed and in a continuous manner as measured over the entire cross-section of the grate trough in the direction of the axis of the conveyor screw. This not only results in a low specific capacity, but also causes inferior or only partial combustion and, thus, low efficiency with a high degree of emissions.
It is an object of the invention to avoid the disadvantages of the prior furnaces and to provide a furnace of simpler construction and with a higher specific capacity, the furnace featuring a more rapid combustion with higher efficiency resulting from use of an air supply which provides an air distribution such as to obtain more intense combustion.
In accordance with the invention, this object is met by arranging the conveyor screw so that it forms a movable fire grate and serves to introduce a portion or all of the combustion air into the combustion chamber of the furnace so as to provide a uniform distribution of the air in the chamber.
The invention replaces the stationary fire grate required in the known underfeed firing constructions by a more advantageous, effectively movable grate in the form of a conveyor screw, or a number of conveyor screws arrranged parallel to one another. As a result, two important functions, namely charging of the fuel and passing it through a fire grate are simultaneously fulfilled. In addition, a third important function is realized. That is, the supply of combustion air, or at least a portion thereof, and its uniform distribution over the entire depth of the combustion chamber, is carried out by the conveyor screw. As already mentioned, this not only intensifies, but also accelerates combustion. Finally, the conveyor screw also serves to remove ash. Accordingly, the furnace according to the invention can be controlled automatically, and in a simple manner. Notably, the enhanced combustion which is now possible significantly reduces the degree of emissions into the exhaust air, without requiring additional filters or similar devices.
In accordance with another aspect of the invention, the conveyor screw or screws extend over the entire depth of the combustion chamber, and the ends of the conveyor screw or screws are supported outside the chamber interior, in or outside of the chamber walls so as to be protected from heat. This simplifies and reduces the cost of the construction of bearings for the conveyor screw and, above all, the service life of the bearings is increased.
In addition to introducing and distributing combustion air through the conveyor screw or screws, the invention contemplates the introduction of additional air for combustion on both sides of and over the length of the grate trough into the combustion chamber. For this purpose, the grate trough is provided, in a known manner, with injection openings for admitting a portion of the combustion air.
This additional injection of combustion air over the length of the grate trough can be used when complete combustion of the fuel material is desired, in spite of an undesirable consistency of the material. However, if only partial combustion is desired in order to produce combustible gases, as in the case of so-called pyrolysis through combustion with a deficiency of the amount of oxygen, the metered supply of combustion air through only the conveyor screw or screws is sufficient.
In accordance with a first embodiment of the invention, a conveyor screw arrangement is constructed with a double wall and consists of a central tube which extends over the depth of the combustion chamber in the direction of the tube axis, and has air transfer openings for passage of the combustion air. At the end of the central tube closer to a combustion air blower, the tube extends through a wind box, and is provided with air inlet windows at this location. The arrangement further includes an outer conveyor tube with a conveyor screw or individually mounted blades for conveying the fuel, the outer conveyor tube being supported coaxially on and fixed for rotation with the central tube at a certain radial distance therefrom. The outer tube has a number of outlet openings, particularly through the blades, for directing air into the combustion chamber. This construction facilitates dynamic cooling, by way of the transferred combustion air, of the outer conveyor tube which is directly exposed to the furnace fire, and provides direct protection of the central tube. Both tubes can be made of suitable materials which are mechanically and thermally compatible with one another. A second embodiment of the invention features a number of individual hubs arranged adjacent one another on the central tube for rotation therewith. The hubs have a U-shaped cross-section and serve as blade carriers. The hubs form individual annular spaces which communicate with the air transfer bores provided in the central tube. Combustion air from these annular spaces is discharged through openings provided in the hubs and/or through nozzles extending radially in the blades.
In a variation of the second embodiment, each hub has two diametrically oppositely located blades, and the air transfer bores provided in the central tube are arranged diametrically opposite each other and angularly offset by 90.degree. relative to the two blades of each hub. In this case, an axialy extending drive spline is fastened on the outer circumference of the central tube, and axially extending grooves are provided on the inner circumferences of any two adjacent hubs so that their blades are angularly offset by 90.degree. relative to one another, the grooves engaging the drive spline on the central tube.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention .